This invention relates to an improved method and apparatus for recovery of metal, particularly aluminum, from dross which has been skimmed from a reverberatory or electric furnace.
Aluminum dross is a combination of aluminum metal, various oxides, nitrates and carbides and constitutes a by-product of an aluminum melting operation. Generally the dross floats on the top of the molten aluminum metal in the furnace. The dross can contain anywhere from 30% to 90% aluminum depending upon the particular processing technique and type of furnace. Dross, therefore, in an aluminum melting operation includes a significant amount of aluminum metal which is considered a valuable resource and which desirably must be recovered.
Heretofore a typical dross recovery system for aluminum melting called for spreading and cooling the dross on a floor surface, for example, an aluminum floor surface. Because this cooling process is rather slow, a great deal of the aluminum metal in the dross is lost due to a thermite reaction, i.e., exothermic oxidation of aluminum metal. Typically 2% of the aluminum metal is lost for each minute of cooling. Thus, a dross which initially has about 70% aluminum metal will decrease to about 40% to 60% aluminum metal after cooling because of losses due to a thermite reaction.
To separate the aluminum metal from cooled and solidified dross, one must next crush and break the dross into an aggregate. The aggregate is then further broken down in a conventional ball mill. At each of these stages a certain amount of the aluminum is lost as dust. Ultimately the dross concentrate is processed in a conventional rotary salt furnace which permits the aluminum metal to separate from the remaining dross material. Further metal losses are observed as a result of the furnace operation due to additional thermite reaction. Also there is loss due to formation of slag or dross in the salt furnace. Ultimately the recovery of aluminum by this process is on the order of 40% to 50% of the original aluminum metal in the dross. Consequently a method and mechanism for improving the recovery of aluminum metal as well as other metals from their dross will have a significant commercial and conservation impact.
A technique for recovery of zinc metal from a dross concentrate is disclosed in Ross et al, U.S. Pat. No. 4,057,232. Ross discloses a method for separating molten zinc from dross by use of a press mechanism which compresses the dross in a ladle and squeezes the free metal through openings in the ladle. This concept of using compression to separate free metal from a molten mixture is also taught in Howard, U.S. Pat. No. 563,769, with repect to separation of the noble metal, silver, from lead bullion. Osborn, in U.S. Pat. No. 2,278,135, discloses a dross press for removing antimony from tin, for example. Kuwano et al, U.S. Pat. No. 4,003,559, teaches an agitating device which is designed to squeeze free metallic zinc from dross. In Kuwano et al, the dross is violently stirred in order to effect the separation of the free metal from the dross.
So far as applicants can determine, however, there are no prior art references or practices which utilize compression techniques or apparatus for the separation of aluminum from dross. The present invention therefore contemplates a method of improving aluminum metal recovery from dross and the special apparatus necessary to effect such improved recovery. The invention also may be applied to the recovery of brass and copper from appropriate dross.